It is well known that a germanium doped silica fibre exhibits photosensitivity, so that the refractive index of the core of the fibre changes when exposed to blue-green light, as demonstrated by Hill et al "Photosensitivity in Optical Waveguides; Application to Reflection Filter Fabrication" Applied Physics Letters Vol. 32 No. 10, 647 (1978). It was later shown that even stronger effects occurred if the core was exposed to ultra-violet radiation close to the absorption peak of a germania-related defect at a wavelength of 240 nm. Reference is directed to G. Meltz et al "Formation of Bragg Gratings in Optical Fibres by Transverse Holographic Method" Opt. Lett. Vol. 14, No. 15 823 (1989). The photosensitive phenomenon is not restricted to germania alone; cerium, europium and erbium: germanium have all shown varying degrees of sensitivity in a silica host optical fibre, but none has been as sensitive of germania. Germanium-boron codoping has also proved highly successful producing large index modulations of the core, of the order of 10.sup.-3 and reference is directed to Y. Duval et al, "Correlation between Ultra-violet-induced Refractive Index Change and Photo-luminance is Gedoped Fibre" Applied Physics Letters, Vol. 61, No. 25, 2955 (1992).
Furthermore, it has been reported that the photosensitivity can be enhanced by hot hydrogen treatment of optical fibres. Reference is directed to G. Meltz et al, "Bragg Grating Formation and Germanio Silicate Fibre Photosensitivity" International Workshop of Photo Induced Self-Orgisation Effects in Optical Fibres SPIE Vol. 1516, p185 (1991). Conventionally, optical fibres are formed by taking a glass tube and exposing the interior thereof to a dopant gas, so as to form a dopant deposit on the interior surface thereof. Thereafter, the glass tube is heated and sintered so as to collapse its interior with the result that the dopant forms a core region through the centre. The effect of the dopant is to raise the refractive index of the central or core region and leave a surrounding cladding region of the lower refractive index. The resulting, collapsed, glass tube is then drawn to produce a fine optical fibre, of reduced diameter.about.120 .mu.m, with a core surrounded by cladding. In a conventional manner, the difference .DELTA.n between the refractive indices of the cladding n.sub.1 and the core n.sub.2 causes light to be guided along the core.
In conventional photosensitive optical fibres, i.e. fibres which have a photosensitive core, it is possible to record so-called refractive index Bragg gratings in the fibres and for a general review, reference is directed to "Photosensitive Optical Fibres: Devices and Applications" Kashyap et al, Optical Fibre Technology 1, 17-34 (1994). In a method described in EP-A-0 668 514, the cladding is rendered photosensitive as well as the core , so that the refractive index grating is recorded in both the core and, to an extent, in the cladding. Also, reference is directed to "Optical fiber design for strong gratings photoimprinting with radiation mode suppression" E. Delevaque et al, Conference on Fiber Communication, Technical Digest Series, Vol 8, No 6, pp 343-346, which discloses an optical fibre with a photosensitive core and a photosensitive intermediate region between the core and the cladding. A refractive index grating is written into the core and the intermediate region, which results in suppression of cladding modes. Photosensitive regions around the fibre core have also been used hitherto for mode matching, as described in U.S. Pat. No. 5,416,863.
Refractive index gratings produced in optical fibres according to these prior recording methods can be used as narrow band reflective filters. One use of the reflective filter is to provide a fibre grating laser, as will now be explained.
It is known that when the core of a silica optical fibre is doped with certain rare earth elements such as erbium or ytterbium, the fibre exhibits optical activity and can be used as an amplifier. The fibre is pumped with optical radiation at a first wavelength so that optical radiation at a second, different wavelength is amplified when passed through the pumped fibre. Such a rare earth doped fibre can be used to provide a laser. The rare earth doped fibre is included in an optical cavity, defined at one end by a refractive index fibre grating formed as aforesaid, spliced to the erbium doped fibre.
It would be desirable to write refractive index gratings in the rare earth doped fibre itself, but this has proved difficult in practice. When the fibre is doped with rare earth elements in its core, the fibres usually have little or no germania therein, so that it is difficult to write gratings in such highly doped fibres, although it has been demonstrated and reference is directed to G. Meltz et al supra. In order to write gratings in rare earth doped fibres, they typically need to be treated with hydrogen. Typically, the fibres are additionally doped with aluminium or phosphorous in order to raise the refractive index of the core. Such fibres exhibit photosensitivity in the core at a wavelength in the region of 193 nm but the photosensitivity is limited as compared with the photosensitivity for a germanium or boron doped core, which exhibits photosensitivity at 244 nm.